Internal combustion engines generally include at least one cylinder and a reciprocating piston within the cylinder connected to a crankcase. The cylinder is generally built with at least one intake (transfer) port and at least one exhaust port formed in the side walls of the cylinder.
During the engine's operation, the reciprocating piston within the cylinder alternately opens and closes the intake and exhaust ports. Initially, the intake port opens in communication with an intake passage and feeds an air/fuel mixture to the cylinder for combustion. Subsequently, the exhaust port opens to allow combusted spent gases to exit through the exhaust port in communication with an exhaust passage. This cycle is repeated.
However, since the exhaust port is fixed in size and location within the cylinder, the amount of time that this port is open for communicating the air/fuel mixture or spent gases with the cylinder bore varies according to the relative speed and position of the piston. For example, at any fixed time with respect to the engine's internal cycle, the piston may be fully blocking, fully opening or partially blocking the ports.
The engine's performance depends on the engine's cycle of opening and closing the exhaust and intake ports. In general, maintaining the exhaust passage open for a longer period improves the engine's performance at higher engine speeds. Conversely, keeping the exhaust passage open for a shorter period improves the engine's performance at lower engine speeds. Accordingly, to optimize the engine's performance, it is desirable to variably control the period during which the exhaust valve or passage is open based on the speed of the engine. Therefore, it is desirable to provide an exhaust valve having a variable valve timing that is based on the current operating conditions of the engine. For example, it is desirable to provide a variable exhaust period based on the current engine speed, load, or throttle position.
Several attempts have been made in the prior art to provide a mechanism for controlling the amount of time that the exhaust port is open. For example, FIG. 1 illustrates an internal combustion engine 10 according to U.S. Pat. No. 4,399,788 to Bostelmann. The Bostelmann ('788) patent teaches the technique of utilizing a restricting member 12 mounted within the cylinder 14 structure for adjusting the exhaust valve position based on the gas pressure within the exhaust passage 18. The positive pressure generated in the exhaust passage 18 is communicated via an orifice 24 in the exhaust passage 18 through a pressure tract 20, whereby it inflates a pressure chamber 22 of the valve control mechanism 26.
The valve control mechanism 26 comprises a flexible diaphragm 28 to which the positive exhaust gas pressure is applied. The diaphragm 28 and restricting member 12 are attached to a rigid cover 34 biased by a return spring 30 that opposes the exhaust gas pressure. Accordingly, when the positive pressure generated in the exhaust passage 18 exceeds the downward force of the return spring 30, the rigid cover 34 lifts and retracts the restricting member 12. The valve port restricting member 12 is thereby adapted to the exhaust passage 18 for varying the opening of the exhaust port 32 from a full flow position to a restricting flow position. The restricting member 12 effectively varies the axial extent of the exhaust port 32 along the axial length of the cylinder 14.
Varying the axial extent of the exhaust port 32 along the axial extent of the cylinder 14 relative to a reciprocating piston (not shown), varies the period for which the exhaust port 32 is open or closed during the engine's 10 cycle. Generally, higher engine speed produces a greater pressure. Accordingly, the restricting member 12 is adjusted to effectively provide a larger exhaust port 32 opening. The valve control mechanism 26 only responds to positive pressure for moving the restricting member 12 in one direction and responds to the spring 30 force for moving the restricting member 12 in the opposite direction.
FIG. 2 illustrates an internal combustion engine, shown generally at 36, similar to the mechanism illustrated in FIG. 1. The engine 36 features a valve control mechanism 38. The pressure is communicated through orifice 62, and is directed by a one-way valve 44, through an electronically operated valve 48 and eventually to a pressure chamber 54. The electronically operated valve 48 is controlled by a CDI ignition unit 50, for example. As the pressure chamber 54 pressurizes and overcomes the spring 60 force, a restricting member 46 is retracted from the exhaust passage 42. Similarly, as the pressure decreases, the restricting member 46 is extended into the exhaust passage 42. The valve control mechanism 38 only responds to positive pressure for moving the restricting member 46 in one direction and responds to the spring 60 force for moving the restricting member 46 in the opposite direction.
The CDI ignition unit 50 provides an RPM activated signal to the electronically operated valve 48 at a predetermined engine RPM. At such time, the valve 48 allows positive pressure to communicate with the exhaust valve control mechanism 38 through a pressure feedline 52. The positive pressure pressurizes the pressure chamber 54. The pressure inflates the flexible diaphragm 56 attached to a rigid cover 58. When the CDI signal is canceled, the electronically operated valve 48 closes and disconnects the pressure feed from the one-way valve 44 to the pressure chamber 54. The force of the return spring 60 then repositions the exhaust valve to its down position.
The electronically operated valves used in Bostelmann ('788) operate by opening only to positive pressures. For example, (FIG. 5A) illustrates a typical CDI valve unit 64 for activating the electronic pressure switch. The valve unit 64 comprises a CDI switch 66 and a one-way solenoid valve 68.
Thus, prior art valve actuation systems comprise exhaust valve control mechanisms that operate only in response to positive pressure signals generated within the crankcase or the exhaust passage, and to the mechanical force of a return spring. Furthermore, the pressure cavities in these systems are flexible on all surfaces except one. Accordingly, negative pressure would have a limited effect due to limited non-moving pressure chamber surfaces. In addition, prior art systems generally must include a return spring, since no other pressure is available to force the valve assembly back into its down position.
Therefore, there is a need in the art for an exhaust valve control mechanism that responds to positive and negative pressure. For example, there is a need for an exhaust valve control mechanism that opens in response to a positive or negative pressure signal and closes in response to the opposite signal. Furthermore, there is a need in the art for an exhaust valve control system having a pressure chamber that is flexible only on one surface, thereby allowing both negative and positive pressures to be utilized. There is also a need in the art for an exhaust valve that does not utilize return springs. Finally, there also is a need in the art for an electrically switchable valve that opens on negative pressures or that closes on positive pressures or vice versa.